Extended travel wire bonding machine

ABSTRACT

An extended travel wire bonding machine that includes a first positioning table movable along an X axis in a first horizontal plane, a second positioning table movable along X and Y axes in a second horizontal plane, the second positioning table being supported on the first positioning table, and a bond head supported on the second positioning table. The wire bonding machine may also include a bonding tool attached to the bond head and a carrier for supporting the leadframe strip under the bonding tool. The first positioning table is moveable for substantially the entire length of the leadframe strip to allow the bonding tool to be successively moved over each semiconductor die on the leadframe strip.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 08/905,183 filed Aug. 4, 1997now U.S. Pat. No. 6,015,079, which is a division of Ser. No. 08/574,156filed Dec. 18, 1995, now U.S. Pat. No. 5,813,590 issued Sep. 29, 1998.

FIELD OF THE INVENTION

This invention relates generally to semiconductor device manufacturingand, more particularly, to a positioning table for use with a machine,such as a wire bonding machine, that moves a work piece in a horizontalplane.

BACKGROUND OF THE INVENTION

During the final stages of the fabrication of many types ofsemiconductor devices or “chips”, a single semiconductor die is mountedwithin a sealed package. A series of leads extend out through thepackage for connection to an external circuit. Electrical contact ismade between conductive pads formed on the face of the die and theexternal leads using very thin metal wires. One end of each of thesewires is bonded to a conductive pad on the die and the other end isbonded to a lead. Generally, the initial components in the packagingprocess are the dies and the leadframe strip. Several individualleadframes are formed in each leadframe strip. Each leadframe supports asemiconductor die for packaging and provides the external leads for thefinished chip. Each leadframe strip is moved as a unit through thevarious stages of the packaging process.

In a conventional packaging process, the dies are attached to mountingpaddles on the leadframe strip. Then, during a wire bonding operation,the bond wires are connected to the conductive bond pads on the die andthe lead fingers on the leadframe strip. The dies, bond wires and innerlead fingers are encapsulated and a trim and form operation separatesthe individual packages on the leadframe strip and bends the outer leadfingers into the proper lead configuration. The present invention isdirected to the wire bonding operation. Wire bonding machines are wellknown in the art. Earlier wire bonding machines were manually operatedby an operator viewing the die through a microscope and positioning thebonding tool over the bonding site. More recently, automated wirebonding machines include optical systems for sensing the location of thebond pads and lead fingers and automatically positioning the bondingtool over each bonding site. The Kulicke & Soffa Model 1488 AutomaticBonder wire bonding machine is representative of the automated wirebonding machines that are now commonly used in semiconductormanufacturing.

During a conventional wire bonding process, and using a conventionalwire bonding machine such as the Kulicke & Soffa Model 1488, a singledie attached to a leadframe on the leadframe strip is held between aclamp and a heat block. The heat block heats the die and the leadframeto a temperature of about 150° C. to about 350° C. A bonding toolmechanically presses the bond wire to a bond pad on the die and then toa bonding site on the appropriate lead finger. The bond wire istypically a fine gold or copper wire that is threaded through thebonding tool. The end of the wire is heated by an electrical dischargeor hydrogen torch to a molten state to form a ball of molten metal onthe end of the wire. The molten ball is pressed by the bonding toolagainst the bond pad on the heated die to mechanically bond the bond padand the wire. The bonding tool is then moved over the bonding site onthe corresponding lead finger and the wire is pressed against the leadfinger to mechanically bond the lead finger and the wire. The bond wireis then tensioned and sheared. This process is repeated for each bondpad on the die.

An important step in the wire bonding operation is the indexing of theleadframe strip. Indexing refers to the process of moving the leadframestrip to position each die one at a time under the bonding tool so thatthe optical sensors can locate reference points on the die and properlyalign the bonding tool over the bond pads and bonding sites on the leadfingers. In a conventional wire bonding machine, each leadframe strip isremoved from a magazine wherein several strips are stored and insertedinto a rail type carrier. Thereafter, the leadframe strip is moved orindexed along the carrier for each successive die wire bondingoperation. After all wire bonding operation cycles are completed, theleadframe strip is moved out of the carrier into a second magazine whereit may be stored or transported for further processing. During eachindexing step, the leadframe strip is engaged by the indexing mechanismand moved along the carrier. The leadframes, which are made of thinmetal sheets, are sometimes damaged during these operations. It would beadvantageous to index the bonding tool to each die on the leadframestrip rather than indexing the leadframe strip to the bonding tool. Thiswould minimize the number of times each leadframe strip must be handledand thereby reduce the risk of damaging the leadframe strips. Inaddition, by moving the bonding tool rather than the leadframe strip,the time required to position each die under the bonding tool could bereduced and overall throughput in the packaging process increased.

The horizontal positioning systems used in conventional bondingmachines, however, do not provide sufficient range of motion or speed toallow for bonding tool indexing. Conventional bonding tool positioningdevices, commonly referred to as an X-Y table, have a limited range ofmotion, typically only about 2 inches along both the X and Y directionsof travel. What is needed is a bonding tool positioning system thatprovides a faster and greater range of motion than conventional systems,at least in the X direction along the line of travel of the leadframestrips, while still maintaining the precision necessary to accuratelyalign the bonding tool over the bond pads and lead finger bonding sites.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention is to increasethe speed and range of motion of the bonding tool positioning system.

It is another object to index the bonding tool to each die on theleadframe strip rather than indexing the leadframe strip to the bondingtool.

It is another object of the invention to minimize the number of timesthe leadframe strips are handled during wire bonding operations andthereby reduce the risk of damaging the leadframe strips.

It is a further object to increase throughput in the device packagingprocess.

These and other objects and advantages are achieved by a device forcontrolling the position of a work piece in a horizontal plane. Thepositioning device includes a first positioning table movable along an Xaxis in a first horizontal plane and a second positioning table movablealong X and Y axes in a second horizontal plane. The second positioningtable is supported on the first positioning table. In one preferredversion of the invention, the positioning device is incorporated into awire bonding machine. The wire bonding machine includes a firstpositioning table movable along an X axis in a first horizontal plane, asecond positioning table movable along X and Y axes in a secondhorizontal plane, the second positioning table being supported on thefirst positioning table, and a bond head supported on the secondpositioning table. The wire bonding machine preferably also includes abonding tool attached to the bond head and a carrier for supporting theleadframe strip under the bonding tool. The first positioning table ismoveable for substantially the entire length of the leadframe strip toallow the bonding tool to be successively moved over each semiconductordie on the leadframe strip.

In another version of the invention, the wire bonding machine includes(1) a bonding mechanism for bonding wires between conductive bond padson the semiconductor dies and lead fingers on the leadframes of aleadframe strip and (2) an indexing mechanism operatively coupled to thebonding mechanism for indexing the bonding mechanism to the dies on theleadframe strip. In one embodiment of this aspect of the invention, theindexing mechanism comprises a first positioning table moveable along anX axis in a horizontal plane, the X axis being parallel to thelongitudinal axis of the leadframe strip, and a means for supporting thebonding mechanism on the first positioning table, wherein the bondingmechanism is indexed to the dies on the leadframe strip by moving thebonding mechanism forward and backward along the longitudinal axis ofthe leadframe strip to position the bonding mechanism over any one ofthe dies. The supporting means preferably consists of a secondpositioning table movable along X and Y axes in a horizontal plane, thebonding mechanism being supported on and moveable by the secondpositioning table for adjusting the position of the bonding mechanismrelative to a semiconductor die or one of its corresponding leadfingers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top down plan view of a typical leadframe strip.

FIG. 2 is a schematic perspective view of the invented extended travelwire bonding machine.

DETAILED DESCRIPTION OF THE INVENTION

A typical leadframe strip 10 used in semiconductor manufacturing isdepicted in FIG. 1. The leadframe strip 10 is adapted to mount severalsemiconductor dies for packaging. Leadframe strip 10 includes diemounting paddles 12 for mounting the individual dies. In addition, theleadframe strip 10 includes parallel spaced strip rails 14, 16 formedwith a pattern of openings 18 for handling by automated machinery. Theleadframe strip 10 also includes an arrangement of lead fingers 20adapted for attachment to the bond pads of a die during the wire bondingprocess. A terminal end 22 of lead fingers 20 will become the externalleads of a completed semiconductor package. From the point in themanufacturing process in which the die mounting paddles 12 are coatedwith adhesive for die bonding, until the point in which the individualsemiconductor packages or chips are separated from the leadframe striprails 14, 16 during a trim and form operation, leadframe strip 10 istreated as a unit in the manufacturing process.

Referring now to FIG. 2, leadframe strip 10 and components of a wirebonding machine 40 are shown schematically during a wire bondingoperation. Multiple semiconductor dies 24 have been attached to the diemounting paddles 12 (shown in FIG. 1) of leadframe strip 10 prior to thewire bonding operation. Wire bonding machine 40 represents generally anyconventional wire bonding machine of the type having an X-Y positioningtable 42 that controls the position of bond head 44 in a horizontalplane. As is known to those skilled in the art of semiconductorpackaging operations, the term “table” as used herein refers to themechanism or system that moves bond head 44 in a horizontal plane. Thistable mechanism or system includes, for example, servo motor driven leador ball screws that push or pull the bond head over a bearing surface,such as surface 43 in FIG. 2. The Kulicke & Soffa Model 1488 wirebonding machine is representative of the automated wire bonding machinesthat are now commonly used in semiconductor manufacturing. In accordancewith the present invention, such a wire bonding machine can be modifiedto extend the travel of the bond head along the X axis and thereby“index” the bond head to each die on the leadframe strip, rather thanindexing the leadframe strip as with conventional machines.

Indexing as used herein refers to the process of positioning each dieone at a time under bonding tool 48 so that optical sensors in bond head44 can locate reference points on the die and properly align the bondingtool 48 over the bond pads and bonding sites on the lead fingers.Conventional wire bonding machines include an indexing assembly thatmoves leadframe strip 10 along carrier 46 to place each die under thebonding tool 48 in bond head 44. Thus, it is said that the leadframestrip 10 is indexed to properly position each die under bonding tool 48.The wire bonding machine of the present invention, by contrast,“indexes” the bond head 44 rather than the leadframe strip 10. That is,bond head 44 is successively moved into position over each die onleadframe strip 10 after wire bonding for each die is completed.

Referring again to FIG. 2, bonding tool 48 is mounted on bond head 44.Bond head 44 is supported on X-Y positioning table 42. X-Y positioningtable 42 precisely controls the movement of bond head 44 in a horizontalplane. The vertical position of bonding tool 48 is controlled by aZ-axis positioning device that is typically constructed within bond head44. These aspects of the wire bonding machine are conventional and wellknown in the art. X-Y positioning table 42 is supported on extended Xaxis positioning table 50. In operation, leadframe strip 10 is placed incarrier 46 with the first die (moving from right to left) positionedgenerally under bonding tool 48. The initial placement of leadframestrip 10 may be made manually or with an automated leadframe indexingassembly, which is a standard feature on most modern wire bondingmachines. Leadframe strip 10 is then heated to about 150° C. to about350° C. on a heat block (not shown) in a conventional manner well knownin the art. The heat block apparatus will typically include die paddlecontact areas adapted to exert a vacuum force on the bottom of the diepaddles to hold the paddle and the die mounted thereon in contact withthe heat block. A camera and monitor or other suitable optical system isprovided to precisely position bonding tool 48 over the bonding sites.Most modern wire bonding machines may be programmed to operate with anautomatic teaching process for precisely locating the bonding tool 48 byoptically sensing the location of the bond pads and lead fingers. Avariety of automatic teaching processes are well known in the art andare included in the automated wire bonding machines noted earlier inthis description. Such automatic teaching processes may include an imageprocessing system for finding the exact position of the bond pads oneach die with respect to the adjacent lead fingers and then causing theX-Y table positioning system to align the bonding tool 48 precisely overeach bonding site.

A bond wire having a molten ball formed at its end, as previouslydescribed, is pressed by bonding tool 48 against the appropriate firstbond pad on the first die to bond the wire to the pad. The bonding tool48 is then moved by the X-Y positioning table 42 to contact thecorresponding lead finger to bond the wire to the lead finger.Thereafter, the wire is sheared and the process is repeated for eachbond pad on the first die. The movement of the bond head 44 during thispart of the wire bond operation is controlled by X-Y positioning table42. The X-Y positioning table provides for very small but precisemovement of bond head 44 to align bonding tool 48 over each bondingsite.

The total travel of a typical X-Y positioning table is only about 2inches along both the X and Y axes. But, such tables will delivermovement to a tolerance of ±0.0001 inches.

Once the bonding of the first semiconductor die is completed, bond head44 and X-Y positioning table 42 are moved to the right along bearingsurface 51 by extended X axis positioning table 50 to position bondingtool 48 generally over the second and successive dies, as illustrated bythe dotted lines in FIG. 2. The total travel of extended X axispositioning table 50 is preferably slightly longer than leadframe strip10, which are typically 6 inches to 12 inches long. Because X-Ypositioning table 42 will precisely align the bonding tool 48 to thebond sites once the die is located generally under the bond head 44,extended X axis positioning table 50 need only deliver movement to atolerance of about ±0.01 inches. Consequently, the range and speed ofmotion of extended X axis positioning table 50 can be made much greaterthan the range and speed of motion of X-Y positioning table 42. Thus,the bond head 44 can be effectively indexed to the leadframe striprather than indexing the leadframe strip to the bond head, as inconventional wire bonding machines. The bonding cycle described above isrepeated for the second die, and so forth until all dies have beenbonded and the bond head 44 is at the far right end of leadframe strip10. Then, the leadframe strip 10 is released from the heat block andremoved from carrier 46. Another leadframe strip 10 is placed in carrier46 and the process continues as before, except that bond head 44 nowmoves from right to left. Alternatively, bond head 44 can be returned toits far left position before loading the next leadframe strip intocarrier 46.

The structure and operation of extended X axis positioning table 50 issimilar to that of conventional X-Y positioning tables. For example, theX-Y positioning table used in the Kulicke & Soffa Model 1488 automaticwire bonder is a table mechanism that consists of X and Y axis slidesthat are pushed or pulled along their respective tracks by individualservo motor driven ballscrews, represented symbolically by cylinders 52in FIG. 2. Preferably, the ballscrew or similar device will have arelatively more coarse thread or the device will operate at higherrotational speeds, or a combination of both, to increase the speed oftranslation of the bond head 44. Cross roller bearings between gibs andsides of each slide allow for the sliding motion. This type of mechanismis suitable for the extended travel X axis positioning table of thepresent invention, as are other conventional X-Y table mechanisms.

In order to provide a wire bonding machine suitable for practicing themethod of this invention, it may be necessary to modify the conventionalheat block (and clamping device, if applicable) to allow all dies on theleadframe strip to be heated without advancing the leadframe strip alongthe carrier. A suitable heat block and clamping device is described inU.S. Pat. No. 5,322,207 entitled Method And Apparatus For Wire BondingSemiconductor Dice To A Leadframe, issued Jun. 21, 1994 and subject tocommon ownership herewith, incorporated herein by reference in itsentirety.

There has been shown and described a new wire bonding machine having anextended travel X axis positioning table. Although the invention hasbeen described in terms of the preferred embodiment, other embodimentsof the inventive concepts disclosed herein are also possible. Forexample, the extended travel positioning table may be made to providemovement along both the X and Y axes. Therefore, the above descriptionshould not be construed to limit the scope of the invention as set forthin the following claims.

What is claimed is:
 1. A wire bonding machine, comprising a bond headmovable in a first direction within a first tolerance and in a seconddirection perpendicular to the first direction within a second tolerancedifferent from the first tolerance.
 2. A wire bonding machine accordingto claim 1, wherein the first tolerance is about 100 times greater thanthe second tolerance.
 3. A wire bonding machine according to claim 2,wherein the first tolerance is about ±0.01 inches and the secondtolerance is about ±0.0001 inches.